1. Field of the Invention
The present invention relates to a wavelength division multiplexing (WDM) passive optical network system, and more particularly to a WDM passive optical network system in which forward and backward channels have the same wavelength.
2. Description of the Related Art
Recently, demand for broadband multimedia services and high-speed and large-capacity Internet services has abruptly increased. In order to provide, to subscribers, such broadband multimedia services and high-speed and large-capacity Internet services, it is necessary to construct a network architecture based on an optical network. Recently, interest in an optical network directly connected to optical network units (ONUs), using optical fibers, has also increased in order to provide broadband services to subscribers.
In order to construct an effective and economical optical network, active research into passive optical networks (PONS) has also recently been conducted. A passive optical network is a system in which a central office (CO), that is, a service provider, and ONUs, that is, service demanders, are connected only by passive optical elements.
In such a PON, typically, the connection between the central office and a remote node installed in an area adjacent to subscribers is achieved using a trunk fiber, whereas the connection between the remote node and each ONU is achieved using a distribution fiber, in order to minimize the total length of optical fibers used in the PON.
Such a PON has various advantages in that it is possible to reduce the initial installation costs while easily carrying out the maintenance and repair of the PON because the total length of optical fibers used in the PON is minimized, and subscribers share passive optical elements. By virtue of such advantages, use of such a PON is greatly increasing. In particular, WDM-PON is being highlighted as a next-generation optical network meeting the information age in future because it can provide a large quantity of information to each subscriber while maintaining a high security and easily achieving an improvement in performance.
FIG. 1 is a schematic diagram illustrating the configuration of a general WDM PON system.
In the WDM PON shown in FIG. 1, different wavelengths λ1 to λN are assigned to respective ONUs 300 by a central office 100 so that the central office 100 can simultaneously transmit data to the ONUs 300 through a single optical communication line. Respective ONUs 300 can also transmit data, using different wavelengths λN+1 to λ2N assigned thereto, respectively.
In order to assign different wavelengths to respective subscribers, this WDM PON should be equipped with light sources respectively adapted to provide different wavelengths corresponding to respective assigned wavelengths. In particular, the central office 100 and ONUs 300 should use, as their light sources, expensive light sources such as distributed feedback laser diodes having a very narrow spectrum width, in order to minimize interference between adjacent wavelengths (channels).
Since such a conventional WDM PON uses light sources having a very narrow spectrum width, it is also necessary to use an additional device such as a temperature stabilizer or a current stabilizer, in order to stabilize oscillating wavelengths. Also, such a conventional WDM PON uses forward and backward channels of different wavelengths. For this reason, it is necessary to install multiplexers and demultiplexers for forward and backward optical signals, respectively. As a result, there is a problem of high system construction costs.
In order to solve this problem, research has been conducted into economically constructing a WDM PON using commercially-available, inexpensive optical elements, and there are some associated research reports.
For example, there is a research report entitled “A low cost WDM source with an ASE injected Fabry-Perot semiconductor laser”, IEEE Photonics Technology Letter, Vol. 12, no. 11, pp. 1067-1069, 2000. This research report discloses a method for economically implementing an optical network system by using an ASE (Amplified Spontaneous Emission) and an inexpensive Fabry-Perot laser diode (F-P LD) as respective light sources of a central office and each ONU. In accordance with this method, an ASE outputted from the central office is injected into the F-P LD of the ONU to lock the output wavelength of the F-P LD at the same wavelength as that of the ASE (Hereinafter, this operation is referred to as “injection locking”.). As a result, the F-P LD can oscillate in a single mode, as a distributed feedback laser diode.
However, this method has a drawback in that the central office should be equipped with a separate light source for generating an ASE.
There is another research report entitled “Upstream traffic transmitter using injection-locked Fabry-Perot as modulator for WDM access networks”, Electronics Letters, Vol. 38, No. 1, pp. 43-44, 2002. This research report discloses a method for economically implementing an optical network system using a distributed feedback laser diode (DFB LD) and an F-P LD as respective light sources of a central office and each ONU. In accordance with this method, the ONU receives an optical signal outputted from the DFB LD to use a part of the received optical signal for signal detection while using the remaining part of the received optical signal for injection locking.
However, this method has a drawback in that the DFB LD used as the light source of the central office is expensive. Thus, the above mentioned methods have drawbacks to be solved.
Meanwhile, the physical topology of an optical network is selected from a ring type, a bus type, and a star type, upon designing the optical network in accordance with an application of the optical network. The concept corresponding to the physical topology of an optical network is a logical topology. This logical topology is also selected from a ring type, a bus type, and a star type in accordance the physical and logical connection states of constitutive elements in the optical network. As compared to other types, the ring type topology has been recognized as exhibiting a satisfactory reliability in backbone networks because it can perform a self-healing function even when system switching occurs due to any disaster or accident.
Early developed WDM ring architectures are unidirectional. In order to implement a bi-directional architecture, using such a WDM ring architecture, therefore, it is necessary to use a double fiber. Recently, research on single fiber bi-directional ring networks has been conducted. In accordance with the research, single fiber bi-directional ring networks are implemented using bi-directional add/drop modules (B-ADMs) of a new type (disclosed in, for example, C. H. KIM et al., “Bi-directional WDM Self-Healing Ring Network Based on Simple Bi-directional Add/Drop Amplifier Modules”; and Y. Zhao et al., “A Novel Bi-directional Add/Drop Module for Single Fiber Bi-directional Self-healing Wavelength Division Multiplexed Ring Networks”).
That is, conventional systems having a self-healing function use a double fiber ring architecture. When system switching occurs due to fiber switching in such a system, the path defined between nodes at opposite ends of the switched fiber in the system is bypassed over the self-healing fiber by an active element. Thus, the switched system can be self-healed.
However, the above mentioned single fiber bi-directional ring networks using B-ADMs is complex and expensive while having a problem in that new type optical elements should be used. Accordingly, it is necessary to develop a ring type WDM PON system capable of having a self-healing function by use of add/drop elements having the same wavelength for forward and backward optical signals, in place of complex optical elements.
Meanwhile, in the case of a transmission network constructed using a WDM PON system, it is necessary to perform an add/drop function at each node of the transmission network. Add/drop elements typically used in a WDM system to perform such an add/drop function operate to drop a wavelength signal of a particular channel, and then to add, to the channel, another signal having the same wavelength as the dropped wavelength signal. Such an add/drop element is widely used for separation and addition of a particular channel in WDM systems. The add/drop element may be implemented as one of various types, for example, a waveguide type, a micro-optic type using a thin film filter, or a fiber type.
Generally, a multi-layer dielectric filter is used for WDM filters of a micro-optic type. That is, such a micro-optic type WDM filter can pass a signal of a particular band while reflecting a signal of another particular band because it employs a multi-layer thin film structure. Also, this filter basically has reversible operation characteristics.
In conventional WDM systems, the operation principle of the 4-port add/drop device is frequently used to separate an optical signal of a particular wavelength from forward or backward optical signals of different wavelengths traveling through an optical signal, or to add the optical signal to the forward or backward optical signals (here, the reflected optical signal may be used for transmission whereas the transmitted optical signal for reception). Thus, the above mentioned conventional WDM-PON systems use different channel wavelengths for up and down-links. Conventional PON architectures are advantageous in the case in which subscribers are concentrated on one area because they use a star topology. However, these PON architectures exhibit less gain in terms of fiber installation costs in the case in which the distance between subscribers is long.
In other words, the star type distribution PON architecture is an architecture capable of considerably reducing the fiber installation costs, as compared to point-to-point systems, under the condition in which it is assumed that subscribers are distributed in a concentrated state. However, this architecture exhibits less gain relating to a reduction in fiber installation costs. In particular, the advantage obtained in accordance with use of the PON architecture having a conventional distribution network type is reduced as the PON architecture is similar to a MAN architecture such as a metro Ethernet, a backbone architecture, or a backbone network. Therefore, it is also necessary to develop a system capable of solving this problem.